The present invention relates to a variable cylinder engine which has a plurality of cylinders and performs a reduced-cylinder operation where one or more of the plurality of cylinders are deactivated.
As variable cylinder engines, JP2010-270701A discloses a variable cylinder engine. Specifically, in JP2010-270701A, in a transition to a reduced-cylinder operation, a control of stopping a lift operation of an intake valve to be deactivated is performed. Therefore, the engine in JP2010-270701A includes a mechanism for stopping the operation of the intake valve (valve stopping mechanism). However, since the mechanism is a hydraulic type, due to an increase in viscosity of oil used in the hydraulic control when an engine coolant temperature is low, a period of time required to stop the intake valve of the cylinder to be deactivated (required period of time for the operation of the valve stopping mechanism) becomes long, and the transition to the reduced-cylinder operation may not be performed smoothly.
Therefore, in JP2010-270701A, when the reduced-cylinder operation is requested, the engine coolant temperature is compared with a predetermined lower limit temperature, and when the coolant temperature is confirmed to be lower than the lower limit temperature, a working angle of the intake valve is reduced and a base circle section of a cam (the section spanning from the part causing the intake valve to close to the part causing the intake valve to open) is designed to be long. Thus, even under a condition where the engine coolant temperature is low, the stop operation of the intake valve is completed within the base circle section and the transition to the reduced-cylinder operation is performed smoothly. Therefore, the frequency of the reduced-cylinder operation increases and fuel consumption performance is improved.
On the other hand, in JP2010-270701A, when the engine coolant temperature is higher than the lower limit value, the control of reducing the working angle of the intake valve is not performed and the lift characteristic of the intake valve is set to be normal. However, when the reduced-cylinder operation is performed in this state, if an engine load is relatively increased and the load on the active cylinder is increased, the temperature of the active cylinder is increased and abnormal combustion may occur. In order to avoid such abnormal combustion, an upper limit load for performing the reduced-cylinder operation may be set low; however, the effect of improved fuel consumption will be reduced in this case.